Multiple line telephone services in a modest subscriber installation may simply include separate lines to which individual telephones and corresponding extensions are connected. A more sophisticated system with a greater number of telephone lines and corresponding telephone stations requires multi-line telephones known as "key phones". Each telephone in the system is connected by a cable to a central control box referred to as a "key service unit" (KSU).
A principal advantage of a key telephone system is that it obviates the need for a PBX system and attendant switchboard operators. A problem with the key telephone system, however, is the required cable to the KSU which, if large, complicates an installation and creates difficult service problems should breaks develop in one or more conductors of the cable. In an attempt to resolve this difficulty, current technology has advanced to smaller cables with an ultimate goal of conducting multiple telephone transactions over a single pair of wires without interference between stations.
Small businesses and particularly home offices are naturally inclined to seek an advantageous appearance of a large organization through their telephone system while avoiding major capital investments or service costs. Telephone systems not having a need for a KSU make this objective readily achievable by providing subscribers with accessible system-type features that significantly enhance customer service and business efficiency with cost-effectiveness. But reliability problems experienced with known KSU-less telephone systems occur when the telephones of the system are power-dependent. A power interruption will therefore shut down the system in whole or at least in part to lessen the overall benefits otherwise achieved.
Intercom calling is a prominent feature of KSU-less systems but may require a dedicated wire pair for communication. Multiplexing may also be used for intercom services but is often restricted to only one of the multiple lines interconnecting the telephones of the system. Since intercom conversations are carried on one line, usually the first line, all of the telephones will require, and be restricted to, the same first line for intercom communications. Although independent intercom groups may be set up by assigning different first lines to different telephones in the system, the problem then is to provide intercom access between groups.